The expanding capabilities of researchers to shape materials on the nanoscale have enabled significant and rapid growth in the development of new nanomaterials-based applications and technologies. The controlled synthesis of metal and metal oxide nanoparticles in the quantum size domain is at the forefront of nanomaterials research due to the fact that the properties of nanoscale materials (e.g. electronic, optical, mechanical, chemical and magnetic properties) not only differ significantly from those of bulk materials, but they become critically dependent on particle size, shape, surface chemistry and inter-particle interactions (Grassian, V. H. J., Phys. Chem. C, 2008, 112, 18303-18313). Metallic silver (Ag) nanoparticles (NPs), in particular, have found use in a broad range of applications such as catalysis (Eising, R., et al., Langmuir, 2011, 50, 9893-9897), electronics (Li, Y. N., et al., J. Am. Chem. Soc., 2005, 127, 3266-3267), biosensing (Zhou, W., Int. J. Nanomedicine, 2011, 6, 381-386), water treatment (Pradeep, T. and Anshup, Thin Solid Films, 2009, 517, 6441-6478), and medicine (Jain, P. K., et al., Acc. Chem. Res., 2008, 41, 1578-1586).
While the antibacterial effects of silver species (in particular, ionic silver) have been known for centuries, in recent years there has been renewed interest in silver in the form of Ag NPs for applications in health care and medicine. This interest is due in part to increasing bacterial resistance to classical antibiotics (Rai, M. K., et al., J. Appl. Microbiol., 2012, 112, 841-852). Ag NPs offer novel modes of action and target different cellular structures compared with existing antibiotics, and have vastly increased reactivity over ionic silver, based on equivalent silver mass content, as a result of their large surface area to volume ratios. Several areas of medical care have already benefitted from the ongoing development of Ag NP-based materials. Applications include Ag NP-based wound dressings (Fong, J. and Wood, F., Int. J. Nanomedicine, 2006, 1, 441-449), Ag NP-based biomaterials for orthopaedics, such as use in artificial joint replacement and bone prostheses (Ren, N., et al., J. Mater. Chem., 2012, 22, 19151-19160), Ag NPs as bactericidal coatings for medical devices (Roe, D., et al., J. Antimicrob. Chemotherapy, 2008, 61, 869-876), and Ag NP incorporation into dental materials (US 2007/0213460).
Silver has a long history of use in preventative dentistry. For instance, silver nitrate (AgNO3) and diamine silver fluoride (Ag(NH3)2F), often referred to simply as AgF, have been used to prevent or arrest carious lesions. However, a recognised undesirable side effect of these products is that they stain tooth structure and tooth-coloured restorations (Knight, G. M., et al., Aust. Dent. J., 2005, 50, 242-245). Suspensions of Ag NP-based materials may offer a unique solution to this problem, as they are non-staining, but have the potential to deliver enhanced antibacterial effects.
The antimicrobial activity of Ag NPs is known to be critically dependent on the dimensions of the particles. Specifically, many studies have revealed that smaller sized particles impart greater antimicrobial activity, on the basis of equivalent silver mass content (Morones, J. R., et al., Nanotechnology, 2005, 16, 2346-2353, and Guzman, M., et al., Nanomed.-Nanotech. Biol. Med., 2012, 8, 37-45)). The origin of this apparent size-dependent effect has been the subject of much investigation, and there are several commonly cited explanations. The first is that under aerobic conditions, Ag NPs of smaller size exert increased bacterial toxicity as a result of increased availability of Ag+ ions on the surface of the particles, due to their higher specific surface areas when compared to larger sized particles. While the specific mechanism of bactericidal action of Ag+ ions is currently not fully understood, it is thought to be related to the inactivation of critical thiol-containing enzymes upon cellular interaction. Additionally, Ag+ is believed to detrimentally affect the replication of DNA in cells treated with AgNO3. Furthermore, experimental evidence has also shown that ionic silver from both Ag NP and AgNO3 sources causes structural and morphological changes in treated cells. The second explanation for the observed particle size dependence of Ag NP antibacterial activity is based on reports of a size-dependent interaction of Ag NPs with bacteria.
The consequence of these key findings is that recently there has been significant emphasis on designing synthetic routes that enable a high level of control over Ag NP size, size distribution and stability in suspension (i.e. no increase in size due to particle aggregation). Many methods have been investigated for the size-controlled synthesis of silver nanoparticles, including electrochemical methods, thermal decomposition, laser ablation, microwave irradiation, sonochemical approaches, and chemical reduction methods.
The chemical reduction method for metal NP synthesis is well-studied and can be carried out under mild conditions. This synthetic approach can be tailored to enable the rational design and development of more advanced functional nanocomposite materials. This typically involves solution-phase chemical reduction of a metal salt and precipitation of the particles within a continuous solvent matrix, forming a colloidal sol. This process is commonly performed in the presence of stabilising molecules (e.g. surfactants, lipids, polymers) in order to prevent unwanted aggregation of nanocrystals, and to control the growth, size and shape of the particles, as well as impart some control over their surface chemistry, functionality and dispersibility in a specific solvent system.
Beyond their role as stabilisers, surfactants and amphiphilic polymers can also act as structure-directing agents and templates. For instance, when present in solution at the time of metal salt reduction, they can direct the growth of nanocrystals and influence the resulting NP morphology by stimulating anisotropic growth and the preparation of faceted NPs of defined, non-spherical shapes (Wiley, B. J., et al., J. Phys. Chem. B, 2006, 110, 15666-15675, and Murphy, C. J., et al., Mrs Bulletin, 2005, 30, 349-355). Furthermore, under certain conditions, these molecules can undergo cooperative association to form various colloidal aggregate structures including micelles, bilayers and vesicles, which can subsequently be used as soft templates for NP synthesis. This strategy is increasingly being used to control the size, size distribution and morphology of individual NPs, as well as the larger NP-containing structures. In a template-based NP synthesis, the outer surface of the colloidal aggregate is typically used to accumulate and sequester synthetic precursors, most commonly metal salts, where they are subsequently chemically reduced, thereby initiating nucleation and growth.
Ionic surfactant micelles form an important class of NP templating structures, as their inherent surface charge imparts colloidal stability, and facilitates surface adsorption of the precursors via electrostatic interactions. A critical property of surfactant solutions is the critical micelle concentration (cmc). This is a property which is known to change significantly for ionic surfactants upon the addition of electrolytes. The form of the surfactant molecules (monomer vs. aggregated), and thus the morphology of the template, critically depends on whether the surfactant concentration used during NP synthesis is above or below the cmc of the surfactant in the presence of the corresponding metal salt.
If the ionic surfactant molecules are present in solution as monomers, then a micellar form of the template structure ceases to exist. To ensure the formation of uniform ionic micelle templates for NP synthesis, the surfactant concentration must be in excess of the cmc (adjusted to account for the presence of a metal salt), but less than the critical concentration which causes a change in shape of the spherical micelles. A template that is uniform in terms of size, shape and surface characteristics is necessary to synthesise nanoscale materials with consistent properties and behavior.
Dental caries is caused by bacterial processes that lead to demineralisation of dental hard tissues resulting from the proton by-production of carbohydrate metabolism. Dental caries is a biofilm process comprising over 700 species of bacteria and archea possibilities which form on the tooth surface, with the colonisation community becoming more complex and the proportions of contributing bacteria changing as the disease progresses and cavitation develops. Historically, the dental profession has used a surgical “drill and fill” approach. Currently, in operative procedures, only significantly damaged carious tissue (infected dentine) is removed. Subjacent to this a further layer of affected dentine is usually retained where bacteria have invariably invaded the dentine tubules.
For placement of tooth coloured composite resin fillings, acid treatment with 37% phosphoric acid is used to demineralise enamel creating micro-porosities for attachment of adhesive resin microtags before the filling is applied. However, to achieve bonding to dentine, which has a much greater organic content, mild acid treatment is followed by application of bifunctional primer molecules, such as hydroxyethyl methacrylate (HEMA), to encourage formation of a hybrid layer within the collagen-mineral matrix. The hydrophilic end of the HEMA molecule interacts with collagen, while the hydrophobic end interacts with composite resin filling material to chemically bind the resin to the tooth. The alternative to composite, amalgam, does not require acid treatment or other sophisticated chemistry as it is merely placed as a space filler in the cavity and is only retained mechanically.
Treating the symptoms by merely cutting away the demineralised tissue, however, does not address the cause of the disease process, leaving the dentition vulnerable to further destruction by protons resulting from bacterial activity. Conventional filling materials do not target the bacterial source of the disease either. Instead, they simply seal the remaining bacteria within the tooth. This prevents decay until the seal provided by the filling is breached, causing re-activation of the bacteria and leading to a recurrence of the infection. Thus, in order to effectively eradicate dental caries, all remaining bacteria must be eliminated.
Applications for disinfecting tooth surfaces currently available include chemical regimes (chlorhexidine, fluoride, iodine, calcium hydroxide, zinc oxide eugenol (ZnOE), hypochlorites, EDTA, peroxide bleaching agents, Carisolv™, ozone) and laser irradiation. All in some way are ineffective, unable to penetrate tooth tissue, have undesirable side effects, or are not cost effective.
The applicant has found that there is a critical “cmc boundary” for systems comprising anionic surfactants and a metal salt precursor, and that this boundary governs the form of the surfactant molecules at a given concentration, and thus the NP template, and also directs the mechanism of formation of surfactant-Ag nanocomposite materials, and the morphology of the final nanostructures.
The applicant has further found that these materials are effective anti-bacterial agents, and that the anti-bacterial activity is greatly enhanced when bacteria are exposed to these materials in the presence of an electric field. The materials are therefore potentially useful for treating or preventing dental caries.
It is therefore an object of the invention to provide a novel material based on an assembly of surfactant-silver nanoparticle aggregates having a number of potential applications one of which is the treatment of bacterial infections, or to at least provide a useful alternative to existing materials.